<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>

<button id="btn1">按钮一</button>
<button id="btn2">按钮二</button>
<button onclick="demo()" id="btn3">按钮三</button>

<script type="text/javascript">
    const btn1 = document.getElementById('btn1');
    btn1.addEventListener('click',()=>{
        alert('按钮1被点击了')
    })
    const btn2 = document.getElementById('btn2');
    btn2.onclick = ()=>{
        alert('按钮2被点击了')
    }

    function demo(){
        alert('按钮3被点击了')
    }
</script>

</body>
</html>